In cases where a patient is to be scanned at multiple timepoints, it is beneficial to register results with a previous dataset to detect changes between scans. Due to differences in the patient or patient pose, the dataset results of the scans being compared may not be spatially aligned. Deformable registration transforms one dataset to match the other dataset voxel by voxel. These three-dimensional deformable registration methods are more likely to succeed if the changes from one scan to the next are minimal. For example, the arms being up in one acquisition and down in another may make registration less reliable.
For more reliable registration, it is advantageous to position the patient in a similar way for a current scan as the patient was positioned for a previous scan. Manual positioning may be inconsistent. Precise alignment across treatments is desired for radiation therapy. In these cases, the patient is fixed during each application in a plastic mold that is fabricated before the first acquisition, or markers are tattooed on the patient skin for aligning with a projected target during each treatment. These approaches are expensive or invasive. In a general scanning situation, especially for a first diagnostic scan, it is not known that a follow-up scan is needed, so expensive or invasive approaches may not be used.